Exhaust gas from internal combustion engines contains, among other things, nitrogen oxides (NOx). These may not easily be depleted in catalytic after-treatment of the exhaust gas since modern internal combustion engines often operate with lean fuel/air mixtures with excess oxygen in order to increase efficiency. Nitrogen oxides which accumulate may not, however, be oxidized in a lean-burn mode, rather are stored in the interim in a nitrogen oxide storage catalytic converter, also referred to as a NOx storage catalytic converter (lean NOx trap, LNT). If the internal combustion engine is operated with a rich fuel/air mixture, the nitrogen oxides stored in the interim are reduced in a LNT to nitrogen and the nitrogen oxide storage catalytic converter is once again free for the storage of nitrogen oxides. LNT and SCR catalytic converters (SCR: selective catalytic reduction) can also be used separately of one another, or in any desired combination.
In order to reduce nitrogen oxides, a reducing agent can be added to the exhaust gas, wherein ammonia is generally used as the reducing agent which is introduced into the exhaust tract in the form of an aqueous urea solution above the nitrogen oxide reduction catalytic converter. The nitrogen oxide reduction catalytic converter can store ammonia in a certain quantity. If the storage function is exhausted or in transient conditions (e.g., fully loaded), ammonia can escape out of the catalytic converter in the event of overdosing. This phenomenon is also referred to as ammonia slip. Since ammonia has a pungent odor and can be noticed even in very small concentrations, this would lead to an odor in the vicinity of the vehicle in the event of overdosing. This situation can be helped by installing an oxidation catalytic converter behind the SCR catalytic converter, which oxidation catalytic converter converts the ammonia into nitrogen and water in the event of ammonia overdosing. A further possibility for preventing what is known as ammonia slip is a larger configuration of the catalytic converter in order to thus obtain a certain storage function. However, these additional structural measures require additional space and are costly.
Therefore, a method to detect and counteract ammonia slip before large quantities can be released into the environment is desired and described herein.
In one example, the issues described above may be addressed by a method for measuring an exhaust NOx concentration, during an injection of a reductant from an injector, via a sensor located between an exhaust side of an engine and the injector, comparing the measured NOx concentration to a baseline value, and determining if ammonia is slipping through a catalyst in response to the measured NOx concentration being greater than the baseline value by a threshold amount. In this way, reductant is conserved without the introduction of a second catalyst.
As one example, exhaust gas downstream of the catalyst is routed back to an engine independent of an EGR demand. In some examples, EGR flow during the injection may be limited to maintain a combustion stability of the engine. As such, if ammonia slips through the catalyst, the EGR may direct the slipped ammonia back to the engine, where it is oxidized during the combustion process, forming NOx. By doing this, a measured exhaust NOx concentration may surpass a baseline NOx concentration, thereby indicating an ammonia slip through the catalyst. In response to the slip, the injector may be deactivated, thereby preventing any further ammonia slippage.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.